osdmap: make replica separate in default crush map configurable

Add 'osd crush chooseleaf type' option to control what the default
CRUSH rule separates replicas across.  Default to 1 (host), and set it
to 0 in vstart.sh.

Fixes: #3785
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>